Scientists reconstruct mammalian ancestor

This month, an international team of scientists, including several Yale faculty members and alumni, unveiled the first evolutionary reconstruction of the earliest placental mammal ancestor.

Humans, and all mammals that give live birth, are descended from this ancestor, the study found. This mammal, which does not yet have a scientific name because it is a simulation rather than a fossil find, may soon receive a “popular name” for easy referencing, said study lead author Maureen O’Leary ’87, a professor of anatomical sciences at Stony Brook University. An article detailing the six-year study was published in the journal Science on Feb. 8.

To visualize the earliest placental mammal, the research team compiled a matrix of over 4,500 “characters” — physical traits that could be observed in living and fossilized mammals. O’Leary said she defined a character as the smallest unit of physical detail that can also be seen on a similar species.

“Any one bone might have hundreds or thousands of characters on it,” she added.

The researchers gathered and compared their data on MorphoBank — a web application pioneered by O’Leary — to evaluate thousands of characters in 86 fossil and living species. When they had completed a spreadsheet with all of their anatomical observations, the researchers combined that data with DNA sequences to produce an evolutionary tree leading back to the placental mammal ancestor. The tree gave scientists a list of characteristics of the mammalian ancestor. After compiling this list, the researchers teamed up with natural history illustrator Carl Buell to create a detailed image of the first placental mammal.

“Literally everything was researched,” Buell said. “Every tooth on that mouth, I drew. I did every cusp they wanted me to.”

Throughout the project, the researchers also generated a surprising hypothesis about the timing of placental mammal evolution. Their data suggests placental mammals did not diversify until 300,000 to 400,000 years after the extinction of non-avian dinosaurs, which occurred about 65 million years ago. Previous estimates, based solely on genetic data, estimated that the ancestor of today’s placental mammals existed over 30 million years before this extinction. John Gatesy GRD ’93, an associate professor of biology at the University of California, Riverside who was not part of the study, said that the hypothesis of rapid mammal diversification is the most controversial aspect of the paper.

“I think this will not be accepted by many geneticists, who instead simply assert that these basal lineages extend deep into the Cretaceous,” Gatesy said in an email.

Study co-author and Yale anthropology professor Eric Sargis said the online tools and unprecedented data set are important contributions of the study. The character matrix that the researchers assembled is much larger than any previous matrix and is now available to researchers and the public through MorphoBank.

“MorphoBank is a huge advance,” Sargis said. “It’s going to make future studies much more explicit and repeatable.”

Sargis said the team’s Science paper marks only the end of the initial stage of their research — the next step is to add more mammal species to the matrix.

“There’s still a lot more work to do, especially with living creatures. Hopefully we can score 500, or 1,000 specimens and see how it changes our view,” Sargis said.

Other Yale-affiliated researchers on the international team include John Flynn ’77, a curator at the American Museum of Natural History, and Jonathan Bloch, an Edward P. Bass Distinguished Visiting Environmental Scholar of Yale’s Anthropology Department.